1. Field of the Invention
The present invention relates to a hydraulic control apparatus for controlling a stepless transmission employed in a vehicle, an industrial machine or the like.
2. Description of the Related Art
There is one type of stepless transmission as shown in FIG. 9 in which an endless belt is stretched between and wrapped around a pair of pulleys having their shafts defined as output and input shafts, respectively, and the width of a groove defined in each pulley is varied to change the radius of wrap of the belt, thereby effecting a change of rotational speed between the pulleys.
This type of stepless transmission will be explained below. A pair of transmission pulleys P1 and P2 are arranged such that cones P3 and P4 of each pulley are moved toward and away from each other in response to a change in volume of the corresponding cylinder C1 or C2, thereby allowing the radius of wrap of an endless belt B to be varied.
The width of the V-shaped groove is adjusted by a first control means V1 for controlling the hydraulic pressure in the cylinder C1 which is integral with the first transmission pulley P1, and the belt tension is controlled by a second control means V2 for adjusting the hydraulic pressure in the cylinder C2 which is integral with the second transmission pulley P2.
Oil which is supplied to a main circuit ML by a hydraulic pressure supply means SP is led to the first and second control means V1 and V2 through first and second main circuits ML1 and ML2. The supply means SP is directly coupled to and driven by a drive source which also drives the input shaft of the stepless transmission.
The pressure of contact between the belt B and the second transmission pulley P2 and the initial tension of the belt B are controlled by adjustment of the hydraulic pressure in the second cylinder C2 effected by the second control means V2. Accordingly, the hydraulic pressure in the cylinder C2 is controlled so as to be equal to or lower than the hydraulic pressure in the main circuit ML, generally as a function of the velocity ratio (i.e. rotational speed ratio). However, in the case where there is a delay in attaining a target velocity ratio, the hydraulic pressure in the second cylinder C2 may be controlled in a different way.
The velocity ratio, that is, the ratio of rotational speed of the output shaft to that of the input shaft, is controlled by the first control means V1 which adjusts the amount of oil supplied to or discharged from the cylinder C1 of the first transmission pulley P1.
The hydraulic pressure in the cylinder C1 of the first transmission pulley P1 is changed in accordance with the level of the hydraulic pressure in the cylinder C2 of the second transmission pulley P2, the output torque, the velocity ratio, and a target shift-up or shift-down speed. Therefore, the pressure in the cylinder C1 may be lower in some cases and higher in the other cases than the pressure in the second cylinder C2, and it is conventional practice to set the hydraulic pressure in the main circuit ML at a constant level which is determined by the highest pressure of those which are required for the cylinders C1 and C2.
Setting the hydraulic pressure in the main circuit ML at a constant level enables simplification of the oil hydraulic system but, at the same time, involves the following problems.
(1) Since the power consumed by the hydraulic pressure supply means SP increases substantially in proportion to the hydraulic pressure in the main circuit ML, the prior art in which the hydraulic pressure in the main circuit ML is constantly used at a relatively high level suffers a large power loss, which means that a disadvantageously large input energy may be required to obtain the same transmission output. In other words, in a vehicle which is driven by an engine, the rate of fuel consumption is increased to bring about an economical disadvantage.
(2) In addition, the bearing load in pump means (not shown) which is an element constituting the hydraulic pressure supply means SP increases in proportion to the level of the hydraulic pressure in the main circuit ML. Therefore, it is necessary to increase the volumetric capacity of the bearing from the viewpoint of the lifetime thereof, which involves a hindrance to education in size and weight of the stepless transmission. When the apparatus is employed in a vehicle or the like, the increase in the weight of the transmission leads to an increase in weight of the vehicle as a whole and hence a rise in the rate of fuel consumption, and the increase in the size of the transmission unfavorably narrows the compartment space in the vehicle, thus bringing about disadvantages in terms of economy and commercial value.